Secondary batteries are devices capable of storing energy in chemical form and of converting into electrical energy to generate electricity when needed. The secondary batteries are also referred to as rechargeable batteries because they can be recharged repeatedly. Common secondary batteries include lead accumulators, NiCd batteries, NiMH accumulators, Li-ion batteries, Li-ion polymer batteries, and the like. When compared with disposable primary batteries, not only are the secondary batteries more economically efficient, they are also more environmentally friendly.
Secondary batteries are currently used in applications requiring low electric power, for example, equipment to start vehicles, mobile devices, tools, uninterruptible power supplies, and the like. Recently, as the development of wireless communication technologies has been leading to the popularization of mobile devices and even to the mobilization of many kinds of conventional devices, the demand for secondary batteries has been dramatically increasing. Secondary batteries are also used in environmentally friendly next-generation vehicles such as hybrid vehicles and electric vehicles to reduce the costs and weight and to increase the service life of the vehicles.
Generally, secondary batteries have a cylindrical, prismatic, or pouch shape. This is associated with a fabrication process of the secondary batteries in which an electrode assembly composed of an anode, a cathode, and a separator is mounted in a cylindrical or prismatic metal casing or a pouch-shaped casing of an aluminum laminate sheet, and in which the casing is filled with electrolyte. Because a predetermined mounting space for the electrode assembly is necessary in this process, the cylindrical, prismatic or pouch shape of the secondary batteries is a limitation in developing various shapes of mobile devices. Accordingly, there is a need for secondary batteries of a new structure that are easily adaptable in shape, and linear batteries have been proposed as a battery having good flexibility and a very high ratio of length relative to cross-sectional diameter.
However, when the linear battery is a cable-type secondary battery, external physical impact such as bending is frequently applied to the battery due to the structural characteristic of the battery, so a short circuit may highly occur. Also, when an anode active material such as Si and Sn is used in the battery, it may be released by the expansion and contraction of the battery during repeated charging and discharging processes. In addition, when an anode active material layer made of a metal becomes thick for the purpose of capacity increase, it is difficult for lithium ions to be distributed in the anode active material layer, thereby resulting in insufficient battery capacity.